There is known a vehicle which includes a main drive power source for driving a main drive wheel through a coupling device and a transmission, an electric generator operated by a drive force generated by the main drive power source, and an electric motor which is driven by electric power generated by the electric generator, to generate a drive torque that can be transmitted to an auxiliary drive wheel. Patent Document 1 discloses an example of such a vehicle.
In this type of vehicle, the electric generator is operated by the drive force generated by the main drive power source provided to drive the main drive wheels, and the electric motor is operated by the electric power generated by the electric generator, to generate the drive torque to be transmitted to the auxiliary drive wheels. This type of vehicle is advantageous in that the vehicle need not be provided with a battery, for driving the auxiliary drive wheels.
Where the vehicle is not provided with the battery, the electric power to be supplied to the electric motor is limited to the electric power concurrently generated by the electric generator driven by the drive force of the main drive power source. The amount of electric power that is generated by the electric generator depends upon the operating speed of the main drive power source used to drive the electric generator. Accordingly, a drop of the operating speed of the main drive power source causes a drop of the amount of electric power generated by the electric generator, which may cause a failure of the electric generator to generate the drive force required to drive the auxiliary drive wheels. In order to prevent insufficiency or failure of generation of the electric power by the electric generator, Patent Document 1 discloses a solution by inhibiting a change of a shifting map to thereby preventing a shift-up action of the transmission, for holding the operating speed of the main drive power source at an adequate value.
To prevent the above-indicated insufficiency or failure of generation of the electric power by the electric generator, Patent Document 2 discloses a technique of increasing an upper limit of a permissible slip ratio of the main drive wheels during starting or acceleration of the vehicle at a running speed of zero or at an extremely low running speed close to substantially zero, so that the main drive wheels are positively slipped, to increase the amount of electric power generated by the electric generator.
Patent Document 1: JP-3552710 B
Patent Document 2: JP-2007-147056 A